1. Field of the Invention
The present invention relates to an optical system and a position detector for positioning a head by using light when recording and/or reproducing information in and/or from an information storage medium such as a high-density floppy disk or optical disk, and a magnetic recorder using the optical system and position detector or an aperture-provided lens for realizing the optical system, position detector, and magnetic recorder.
2. Related Art of the Invention
As recording information in a magnetic disk such as a floppy disk at higher density, the interval between tracks each of which is a string of pieces of information decreases. Therefore, because positioning of a magnetic head in the direction perpendicular, to tracks is difficult at a mechanical accuracy, an art for performing positioning by light has been developed.
For example, when the interval between guide grooves constituted with a pit string for tracking a high-density floppy disk is approx. 20 .mu.m, it is possible to obtain a track error signal by using an optical system having a light-source wavelength of 780 nm and a numerical aperture of NA=approx. 0.04 at the disk side.
A conventional position detector in the track direction of a high-density floppy disk is described below by referring to FIG. 23.
As shown in FIG. 23, a light beam emitted from a semiconductor laser 101 serving as a light source produces a zero-order light ray and positive and negative first-order diffracted-light rays by a diffraction device 102 though not illustrated. In this case, the zero-order light ray is referred to as a main beam and the positive and negative first-order diffracted-light rays are referred to as sub-beams. The main beam and two sub-beams pass through a half mirror 103 and are condensed by a lens 104.
Openings of the main beam and sub-beams condensed by the lens 104 are restricted by an aperture 109 so that the openings have a predetermined numerical aperture NA and the main beam and sub-beams are condensed to a floppy disk of an information storage medium (-hereafter referred to as disk) 107.
Concentric information data strings are recorded in the disk 107 and they are referred to as tracks. Moreover, guide grooves are formed between tracks at proper intervals on the disk 107 so that positioning can be made by light rays. A beam string constituted with the main beam and two sub-beams is arranged so as to form a predetermined angle to the guide grooves on the disk 107.
Light rays reflected from the disk 107 pass through the aperture 109 and lens 104 again and they are reflected by the half mirror 103 to enter a photodetector 108.
The photodetector 108 is constituted with a plurality of detection regions, which receives three beams of the main beam and two sub-beams separately from each other and outputs a signal corresponding to the received luminous energy. These three beams irradiate respectively relatively different positions in the direction perpendicular to the guide grooves on the disk 107 on the basis of the guide grooves. Therefore, modulation factors of signals obtained from three detection regions are different from each other. By inputting these signals to an arithmetic circuit 300 and computing them, it is possible to detect the relative positional relations between guide grooves and beam irradiating positions. By using the relations, a magnetic head 201 is positioned to a track on the disk 107 to record and reproduce information into and from a desired track. Description of the arithmetic method by the arithmetic circuit 300 is omitted because the method is described in the official gazette of Japanese Patent Application Laid-open No. 9-161424 in detail.
Moreover, though not illustrated, there is another conventional position detector without a half mirror, having a structure in which condenser lenses are set to the optical path of the light emitted from a semiconductor laser and the optical path of the light reflected on a disk one each in order to separate the both optical paths from each other and light is diagonally applied to the disk 107.
However, this conventional type of the position detector has a problem that, when a disk tilts from a light beam, some of a light beam reflected on the surface of a disk is interrupted on a returning path and thereby, a position detection signal cannot be stably detected.